The present invention relates to a steering mechanism for vehicles, and particularly to vehicles employing a sharp turning radius. The present invention also relates to steering mechanisms for vehicles which employ a pair of independently controlled driving wheels.
A variety of motor driven vehicle applications require that the vehicle be capable of negotiating sharp turns, such as a garden tractor or a riding lawnmower. In vehicles of this sort, a pair of drive wheels powered by a small horsepower motor provide the motive force for the vehicle. In order to provide steerage for the vehicle, the driving wheels are provided with independent controls and with independent hydrostatic or variable speed mechanical transmissions.
In one example of a vehicle of the type described, a steering mechanism, such as that described in the patent to Tsuji, et al., U.S. Pat. No. 4,154,314, provides independent control of the speed of each of the driving wheels, such that in negotiating a turn, one wheel is slowly braked while the speed of the other wheel is increased. The Tsuji, et al. steering mechanism uses a control lever corresponding to each of the driving wheels. Lever-type controls of this nature have several problems - for instance, they may be difficult to control around the neutral position, that is, a hopping action may result around neutral when the operator and/or the control lever is moving in one direction and the vehicle is moving in the opposite direction. Another problem is that a typical consumer using a riding lawn mower may find it confusing to use a lever that controls both the vehicle speed and the steering. A steering wheel is more natural to the average consumer who has not had much exposure to machines controlled by levers. Finally, although the Tsuji, et al. device provides for a small turning radius, it does not provide for a zero turning radius - that is, a vehicle turn effectively made about the midpoint of the driving wheel axis.
One device described in a patent to Davis, et al., U.S. Pat. No. 3,362,493, provides the capability of performing a zero turn radius by the vehicle. However, the Davis, et al. device is a complicated assemblage of cams, levers and linkages. In addition, a separate forward-reverse direction control is required to change the direction of motion of the vehicle. Finally, although the Davis, et al. device can produce a zero turning radius, there is no provision in Davis, et al. for a reduction in vehicle speed as the vehicle steerage is increased. That is, when the vehicle is moving at a zero turning radius, there is no provision for reducing or limiting the speed the vehicle moves around the turn, thus creating a risk of tipping the vehicle during a sharp turn.